Apparatus for driving the arc in a cathodic arc coater

ABSTRACT

An apparatus for applying material by cathodic arc vapor deposition to a substrate is provided which includes a vessel, apparatus for maintaining a vacuum in the vessel, a disk-shaped cathode, apparatus for selectively sustaining an arc of electrical energy between the cathode and an anode, and apparatus for driving the arc around an axially extending evaporative surface of the cathode. The apparatus for driving the arc includes a magnetic field generator attached to a ferromagnetic center piece. The magnetic field generator includes a plurality of side magnets attached to the ferromagnetic center piece, and a center magnet positioned radially inside of the side magnets. Each side magnet produces a magnetic field that permeates the cathode, and each magnetic field includes a portion that runs substantially parallel to the evaporative surface. The center magnet influences the axial position of the arc path relative to the evaporative surface.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to apparatus for vapor deposition of coatings ingeneral, and to cathodic arc vapor deposition apparatus in particular.

2. Background Information

Vapor deposition as a means for applying a coating to a substrate is aknown art that includes processes such as chemical vapor deposition,physical vapor deposition, and cathodic arc vapor deposition. Chemicalvapor deposition involves introducing reactive gaseous elements into adeposition chamber containing one or more substrates to be coated.Physical vapor deposition involves providing a source material and asubstrate to be coated in a evacuated deposition chamber. The sourcematerial is converted into vapor by an energy input, such as heating byresistive, inductive, or electron beam means.

Cathodic arc vapor deposition involves a source material and a substrateto be coated placed in an evacuated deposition chamber. The chambercontains only a relatively small amount of gas. The negative lead of adirect current (DC) power supply is attached to the source material(hereinafter referred to as the "cathode") and the positive lead isattached to an anodic member. An arc-initiating trigger, at or near thesame electrical potential as the anode, contacts the cathode andsubsequently moves away from the cathode. When the trigger is still inclose proximity to the cathode, the difference in electrical potentialbetween the trigger and the cathode causes an arc of electricity toextend therebetween. As the trigger moves further away, the arc jumpsbetween the cathode and the anodic chamber. The exact point, or points,where an arc touches the surface of the cathode is referred to as acathode spot. Absent a steering mechanism, a cathode spot will moverandomly about the surface of the cathode.

The energy deposited by the arc at a cathode spot is intense; on theorder of 10⁵ to 10⁷ amperes per square centimeter with a duration of afew to several microseconds. The intensity of the energy raises thelocal temperature of the cathode spot to approximately equal that of theboiling point of the cathode material (at the evacuated chamberpressure). As a result, cathode material at the cathode spot vaporizesinto a plasma containing atoms, molecules, ions, electrons, andparticles. Positively charged ions liberated from the cathode areattracted toward any object within the deposition chamber having anegative electrical potential relative to the positively charged ion.Some deposition processes maintain the substrate to be coated at thesame electrical potential as the anode. Other processes use a biasingsource to lower the potential of the substrate and thereby make thesubstrate relatively more attractive to the positively charged ions. Ineither case, the substrate becomes coated with the vaporized materialliberated from the cathode.

The random movement of the arc can sometimes lead to nonuniform erosionof the cathode, which in turn can limit the useful life of the cathode.To avoid non-uniform erosion, it is known to steer the arc relative tothe cathode. U.S. Pat. Nos. 4,673,477, 4,849,088, and 5,037,522 areexamples of patents that disclose apparatus for steering an arc relativeto a cathode. Some prior art steers the arc by mechanically manipulatinga magnetic field source relative to the cathode. Other prior art steersthe arc by alternately electrically connecting a power supply leadbetween two ends of a cathode. In both these approaches, the speed ofthe arc relative to the cathode is limited by the speed of the apparatusmanipulating the magnetic field source, or switching the power supply.Another limitation is the complexity of the switching mechanisms and thehardware neccesary to manipulate a magnetic field source relative to thecathode. A person of skill in the art will recognize that a productioncoating environment is harsh and simplicity generally equates withreliability.

Presently available cathodic arc coaters typically use a cooled cathodefixed in place within the coater. One cooling scheme provides a manifoldattached to the cathode that permits the passage of coolant between thecathode and manifold. Another scheme uses coolant piping connected to ahollow cathode. A problem with either scheme is that the cathode must bemachined to accept the manifold or piping. Not all cathode materials areamenable to machining and even where possible, machining addssignificantly to the cost of the consumable cathode. Another problemwith "directly cooling" the cathode is the labor required to replace thecathode when its useful life has expired. In the previous example wherea manifold (or piping) is mechanically attached to the cathode, themanifold (or piping) must be detached from the old cathode and attachedto a new one, and the deposition chamber subsequently cleaned ofcoolant. For those applications which require cathode replacement aftereach coating run, the labor costs and down time can be considerable.Still another problem with direct cathode cooling is leakage. Coolantleakage occurring during deposition can contaminate the substrates beingcoated and require extensive cleaning within the deposition chamber.Airfoils for gas turbine engines are an example of an expensivesubstrate to be coated; one where it would be a distinct advantage tominimize or eliminate losses due to contamination.

In short, what is needed is an apparatus for cathodic arc vapordeposition of material on a substrate that operates efficiently, onecapable of consistently providing a high quality coating on a substrate,one that optimizes cathode erosion, and one that operates costeffectively.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the present invention to provide anapparatus for cathodic arc vapor deposition of material on a substratethat operates in a cost effective manner.

It is another object of the present invention to provide an apparatusfor cathodic arc vapor deposition of material on a substrate thatprovides a uniform high quality coating on every substrate within theapparatus.

It is another object of the present invention to provide an apparatusfor cathodic arc vapor deposition of material on a substrate thatincludes apparatus for steering an arc relative to a cathode thatovercomes the limitations of the prior art.

According to the present invention, an apparatus for applying materialby cathodic arc vapor deposition to a substrate is provided whichincludes a vessel, means for maintaining a vacuum in the vessel, adisk-shaped cathode, means for selectively sustaining an arc ofelectrical energy between the cathode and an anode, and means forsteering the arc around the cathode. The arc of electrical energyextending between the cathode and the anode liberates a portion of thecathode which is subsequently deposited on the substrate located insidethe vessel.

An advantage of the present invention is that the present inventionapparatus for cathodic arc vapor deposition of material on a substrateis designed to operate in a cost effective manner. One cost effectivecharacteristic of the present invention is the cathode. The presentinvention cathode is preferably disk-shaped and can be cut, for example,from a cylindrical casting. The simply formed cathode requires minimalexpensive machining, thereby reducing the cost of the cathode and theoverall coating process. Another cost effective characteristic is thatthe erosion of the cathode is circumferentially uniform. As a result,the life of the cathode can be maximized before replacement isnecessary.

Another advantage of the present invention is uniformity of the coatingprocess. The means for steering the arc around the cathode increases theuniformity of the cathode erosion. Specifically, steering the arc aroundthe circumference of the cathode at a substantially constant velocitycauses uniform circumferential erosion of cathode. The substratesdisposed around and equally spaced from the cathode consequently receivea more uniform deposition of coating. In addition, the velocity of thearc around the cathode circumference is a function of the strength ofthe magnetic field and the amount of current supplied. As a result, thevelocity of the arc can be manipulated by changing the amount ofcurrent, the strength of the magnetic field, or both.

Another advantage of the present invention is the simplicity andreliability of the means for steering the arc around the cathode. Themeans for steering the arc includes a magnetic field generator having aplurality of side magnets and a ferromagnetic centerpiece; no switchingmechanism is required. The absense of a switching mechanism increasesthe reliability of the steering means.

These and other objects, features and advantages of the presentinvention will become apparent in light of the detailed description ofthe best mode embodiment thereof, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of the present invention cathodic arcvapor deposition apparatus.

FIG. 2 is a diagrammatic cross-sectional view of a contactor, showingtwin magnetic field generators.

FIG. 3 is a diagrammatic side view of a magnetic field generator.

FIG. 4 is a top view of the magnetic field generator shown in FIG. 3.

FIG. 5 is a diagrammatic top view of a magnetic field generator havingannular side magnets.

FIG. 6 is a side view of the magnetic field generator shown in FIG. 5.

FIG. 7 is a diagrammatic view of a cathode and magnetic field generatorwith a center magnet, including magnetic field lines permeating thecathode.

FIG. 8 is a diagrammatic view of a cathode and magnetic field generator,including magnetic field lines permeating the cathode.

BEST MODE FOR CARRYING OUT THE INVENTION

I. Apparatus

Referring to FIG. 1, an apparatus 10 for cathodic arc vapor depositionon a substrate 12, hereinafter referred to as a "cathodic arc coater",is provided having a vessel 14, means 16 for maintaining a vacuum in thevessel 14, a cathode 18, a contactor 20, and means 22 for sustaining anarc of electrical energy between the cathode 18 and an anode 24. Acoolant supply 26 maintains the apparatus 10 within acceptabletemperatures by cycling coolant through cooling passages 28,30 withinthe vessel 14 and contactor 20. In the preferred embodiment, the means16 for maintaining a vacuum in the vessel 14 includes a mechanical roughvacuum pump 32 and a high volume diffusion-type vacuum pump 34 piped tothe interior of the vessel 24. Other vacuum means may be usedalternatively.

Referring to FIGS. 1 and 2, the cathode 18 is a substantiallycylindrical disk having an evaporative surface 36 extending between apair of end surfaces 38,40. The end surfaces 38,40 are substantiallyparallel with one another. The coating to be deposited dictates thematerial composition of the cathode 18, and often the cathode 18 cansimply be cut from a cast rod. The axial length 42 of the cathode 18 ispreferably longer than the anticipated final width 44 of the erosionpattern 46 along the evaporative surface 36 of the cathode 18. Keepingthe erosion pattern 46 between the end surfaces 38,40 minimizes thepossibility that the arc will leave the evaporative surface 36 of thecathode 18. The substrates 12 are mounted on a platter 48 that rollsinto and out of the vessel 14. The platter 48 includes means forrotating the substrates (not shown).

The contactor 20 includes a head 52 attached to a shaft 54. The head 52is positioned inside the vessel 14 and the shaft 54 extends from thehead 52 to outside the vessel 14. An insulative disk 56 electricallyinsulates the contactor 20 from the vessel 14. The contactor 20preferably further includes a cooling tube 58 coaxially positionedwithin the shaft 54, a coolant inlet port 60 (see FIG. 1) connected tothe cooling tube 58, and a coolant exit port 62 connected to the passage30 formed between the coaxial coolant tube 58 and shaft 54. The coaxialarrangement between the cooling tube 58 and the shaft 54 allows coolantfrom the coolant supply 26 to enter the cooling tube 58 and return viathe passage 30 between the shaft 54 and the cooling tube 58, or viceversa.

Referring to FIGS. 2-8, the contactor head 52 includes a cup 66, a shaftflange 68, and a magnetic field generator 74. The shaft flange 68 isfixed to the shaft 54 and the cup 66 is removably attached to the shaftflange 68. The cup 66, shaft flange 68, and shaft 54 are fabricated froman electrically conductive material such as a copper alloy. The magneticfield generator 74 includes a ferromagnetic center piece 82, a pluralityof side magnets 84, and a center magnet 86. The center piece 82 includesat least one side surface 88 extending between two end surfaces 90, anda hollow 92 for receiving the center magnet 86. The side 84 and center86 magnets are preferably permanent magnets, although alternativemagnetic field sources such as electromagnetics may be used. Cylindricalpermanent magnets, for example, are advantageous because they arecommercially available and relatively inexpensive (see FIGS. 2-4, 7, and8). An annular magnet, segmented or whole, may be used alternatively(see FIGS. 5 and 6). The side magnets 84 are attached to the sidesurface 88 and the center magnet 86 is either received within the hollow92 or attached to an end surface 90. FIG. 8 shows a magnetic fieldgenerator 74 that includes a ferromagnetic center piece 82 and sidemagnets 84, but does not include a center magnet 86. In all embodiments,the number of side magnets 84 can be varied to accommodate the processat hand.

Referring to FIGS. 1 and 2, apparatus 94 is included for rotating themagnetic field generator. The rotation apparatus 94 includes a rod 96extending through the coolant tube 58 and into the head 52 where itconnects with the ferromagnetic center piece 82. The opposite end of therod 96 is connected to a variable speed drive motor 98 via a drive belt100 (see FIG. 1).

The cathodic arc coater 10 may also include an actuator 102 forselectively actuating the contactor 20 into electrical contact with thecathode 18 which includes a pair of two-way actuating cylinders 104(hydraulic or pneumatic) acting between the vessel 14 and a shaft flange106 attached to the contactor shaft 54. Mechanical apparatus may be usedin place of the actuating cylinders 104. A commercially availablecontroller (not shown) can be used to control the position and force ofthe cylinders 104 (or mechanical apparatus). FIG. 1 shows the cathode 18disposed between a fixed support 19 and an actuable contactor 20. FIG. 2shows an alternative cathode 18 arrangement wherein a contactor 20 is incontact with each end surface 38,40 of the cathode 18. The secondcontactor 20 may be fixed or actuable.

The cathodic arc coater 10 preferably includes a biasing source 108 forelectrically biasing the substrates 12. Negatively biasing thesubstrates 12 relative to the anode 24 makes the substrates 12electrically attractive to positive ions liberated from the cathode 18.A contact 110 electrically connects the biasing source 108 to theplatter 48. The substrates 12, which are electrically connected to theplatter 48, are consequently electrically connected to the biasingsource 108. Other means for electrically connecting the substrates 12 tothe biasing source 108 may be used alternatively.

Deflector shields 112 are used throughout the coater 10 to confine thevaporized cathode materials in the area of the substrates 12. Thedeflector shields 112 attached to the vessel 14, platter 48, andcontactor 20 also minimize undesirable material build-up on thosesurfaces. In the preferred embodiment, the deflector shields 112attached to the vessel 14 are electrically connected to the vessel 14and are made of an electrically conductive material resistant tocorrosion, such as stainless steel.

The means 22 for sustaining an arc of electrical energy between thecathode 18 and an anode 24 includes a direct current (D.C.) power supply114. In the preferred embodiment, the positive lead 115 of the powersupply 114 is connected to the vessel 14, thereby making the vessel 14act as an anode. The negative lead 117 of the power supply 114 iselectrically connected to the contactor shaft 52. Alternativeembodiments may use an anode (not shown) disposed inside the vessel 14.An arc initiator 116, maintained at or near the electrical potential ofthe vessel 14, is used to initiate an arc.

II. Operation of the Apparatus

Referring to FIG. 1, in the operation of the present invention cathodicarc coater 10, a plurality of substrates 12 and a cathode 18 areattached to the platter 48 and loaded into the vessel 14. The substrates12 have been previously degreased and substantially cleaned, althougheach will likely have some molecular contaminant and oxidation remainingon its exterior surface. The actuating cylinders 104 subsequentlyactuate the contactor 20 into electrical contact with the cathode 18 andthe vessel 14 is closed.

The mechanical rough vacuum pump 32 is operated to evacuate the vessel14 to a predetermined pressure. Once that pressure is reached, the highvolume diffusion vacuum pump 34 further evacuates the vessel 14 to nearvacuum conditions. The substrates 12 are then cleaned of any remainingcontaminants and/or oxidation by a method such as "sputter cleaning".Sputter cleaning is a process well known in the art and will not bedescribed in detail here. Other cleaning methods may be usedalternatively. After the substrates 12 are cleaned, the contaminants arepurged typically using an inert gas.

Prior to initiating an arc several steps are completed, including: (1)the substrates 12 are electrically biased via the biasing source 108,making them electrically attractive to positive ions emitted from thecathode 18; (2) the substrates 12 are rotated at a particular rotationalspeed; (3) the power supply 114 is set to establish an arc having aparticular magnitude of current and voltage, but no arc is initiated;(4) the vacuum pumps 32,34 establish and maintain a particular vacuumpressure of gas within the vessel 14; and (5) coolant is cycled throughthe cooling passages 28,30 within the vessel 14 and contactor 20.Specific process parameters will depend upon factors such as thesubstrate material, the material to be coated, and the desiredcharacteristics of the coating, etc.

Once the aforementioned steps have been completed, the arc initiator 116is brought into and out of contact with the evaporative surface 36 ofthe cathode 18, causing an arc to jump between the arc initiator 116 andthe evaporative surface 36. The arc initiator 116 is subsequently moveda distance away from the cathode 18, preferably radially outside of thesubstrates 12. Once the arc initiator 116 is no longer proximate thecathode 18, the arc jumps between the cathode 18 and the deflectorshields 112 electrically connected to the vessel 24 (or directly betweenthe cathode 18 and the vessel 24).

The magnetic field generator 74 positioned in the contactor 20 drivesthe cathode spot along the evaporative surface 36 of the cathode 18. Tobe more specific, each side magnet 84 produces a magnetic field thatpermeates the cathode 18 and runs substantially parallel to the cathodeevaporative surface 36. FIGS. 7 and 8 show an approximation of where themagnetic field lines are believed to run and vector 124 represents themagnetic field extending between the cathode end surfaces 38,40. Thedirection of the magnetic field vector 124 depends upon the orientationof the side magnet 84 poles, and all side magnets 84 are oriented inlike manner. A vector 126 representing the electric arc, in contrast,extends away from the evaporative surface 36 in a substantiallyperpendicular direction. Together, the magnetic field and the electriccurrent of the arc create a force (the Hall effect) on the arc thatcauses the arc to travel a distance around the circumference of thecathode 18. The dwell time of the arc at any particular cathode spot isinversely related to the Hall effect force; i.e., an increase in theHall effect force, causes a decrease in the dwell time. A person ofskill in the art will recognize that decreasing the dwell time reducesthe occurrence of macro-particles which can adversely effect theuniformity and surface finish of the deposited coating.

The individual magnetic fields of the side magnets 84, in closecircumferential proximity to one another, collectively force the arc tocircle the cathode evaporative surface 36 along an arc path 122 (seeFIG. 2). The number of side magnets 84, the relative spacing of magneticfields emanating from side magnets 84, and the intensity of thosemagnetic fields can be adjusted to satisfy the application at hand. Insome applications, however, it is advantageous to further include acenter magnet 86. The magnetic field of the center magnet 86 appears toinfluence the geometry of the magnetic fields emanating from the sidemagnets 84. As a result, the arc path 122 around the circumference ofthe cathode 18 is moved axially away from the side magnets 84. Hence,the center magnet 86 can be used to move the axial position of the arcpath 122. FIG. 7 shows an approximation of how the magnetic field linesfrom the side magnets 84 and center magnet 86 are believed to interact.An arc path 122 proximate the axial midpoint of the cathode 18 helpsmaintain the arc away from the contactor 20 (or between both contactors20), thereby minimizing undesirable wear on the contactor 20. An arcpath 122 (and consequent erosion geometry) proximate the axial midpointalso helps increase the efficiency of the coating process by maximizingthe amount of material that can be eroded from a particular cathode 18.

The energy delivered by the arc causes the material at the cathode spotto vaporize, thereby liberating atoms, molecules, ions, electrons, andparticles from the cathode 18. The biased substrates 12 attract theions, causing them to accelerate toward the substrates 12. The ionsstrike the exterior surface of the substrates 12, attach, andcollectively form a coating of the cathode material.

FIG. 2 shows an eroded cathode 18 in phantom, substantially symmetricalabout the arc path 122. In the embodiment which includes apparatus 94for rotating the magnetic field generator 74, rotation of the magneticfield generator 74 within the contactor 20 helps promote uniform axialand circumferential erosion of the cathode 18. The rotation distributesthe magnetic contribution of each side magnet 84 around thecircumference of the cathode 18 as a function of time. It must beemphasized, however, that rotation of the magnetic field generator 74 isnot required to create a circling arc. As stated above, the individualmagnetic fields of the side magnets 84 collectively force the arc tocircle the cathode evaporative surface 36.

Referring to FIG. 1, when a coating of sufficient thickness has beendeposited on the substrates 12, the power supply 114 is turned off andthe arc extinguished. The vessel 14 is purged with inert gas and broughtto ambient pressure. The contactor 20 is actuated out of contact withthe cathode 18 and the platter 48 is removed from the vessel 14. Thesubstrates 12 are subsequently removed from the platter 48 and newsubstrates 12 attached. The loaded platter 48 is then inserted back intothe vessel 14 in the manner described earlier and the process repeated.

Although this invention has been shown and described with respect to thedetailed embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and the scope of the invention. In oneexample, means for cooling the contactor 20 may be used other than thatdisclosed in the Best Mode. In another example, a power supply 114 thatsimulates direct current, such as one having an LCR circuit or otherrectifying means, may be used in place of a D.C. power supply 114. Inanother example, the Best Mode discloses one or both contactors 20 asactuable. In some cases, however, it may be acceptable to havestationary contactors 20.

I claim:
 1. An apparatus for applying material by cathodic arc vapordeposition to a substrate, comprising:a vessel; means for maintaining avacuum in said vessel; a disk-shaped cathode, positioned inside saidvessel, having a first end surface, a second end surface, and anevaporative surface axially extending therebetween; means forselectively sustaining an arc of electrical energy between saidevaporative surface and an anode; means for driving said arc around saidevaporative surface along an arc path, having a magnetic field generatorattached to a ferromagnetic center piece, wherein said magnetic fieldgenerator includes a plurality of side magnets attached to saidferromagnetic center piece, and a center magnet positioned radiallyinside of said side magnets; wherein each said side magnet produces amagnetic field that permeates said cathode, each said magnetic fieldincluding a portion that runs substantially parallel to said evaporativesurface; and wherein said center magnet influences said arc pathrelative to said axially extending evaporative surface; and wherein saidarc of electrical energy extending between said cathode and said anodeliberates a portion of said cathode which is subsequently deposited onthe substrate located inside said vessel.
 2. An apparatus according toclaim 1, wherein said means for sustaining an arc of electrical energybetween said cathode and said anode comprises:a power supply, having apositive lead and a negative lead; wherein said negative lead of saidpower supply is electrically connected to cathode, and said positivelead is electrically connected to said vessel, thereby making saidvessel act as said anode; and wherein said cathode is electricallyinsulated from said vessel.
 3. An apparatus according to claim 2,wherein said magnets are permanent magnets.
 4. An apparatus according toclaim 3, wherein said ferromagnetic center piece comprises a hollow andsaid center magnet is disposed in said hollow.
 5. An apparatus accordingto claim 4, wherein said center magnet is cylindrically shaped, having afirst polar end and a second polar end, said second polar end having apolarity opposite that of said first polar end;wherein said first polarend of each said magnet is are attached to said ferromagnetic centerpiece.
 6. An apparatus according to claim 5, further comprising:aprotective coating, said coating encasing said ferromagnetic centerpiece and said magnets to inhibit corrosion of said ferromagnetic centerpiece and said magnets.
 7. An apparatus according to claim 3, whereinsaid center magnet is attached to an end surface of said ferromagneticcenter piece.